Khan Academy
Khan Academy: Absorption/emission Lines
An article discussing why absorption lines of Fraunhofer lines appear.
Other
Planetary Nebulae Observer Home Page
This home page is linked to detailed descriptions of nebulae types, shapes, and structures and their relationship to stellar evolution, a historical perspective on the discovery of nebulae, a gallery of images, and other related resources.
Wolfram Research
Wolfram Science World: Gustav Kirchhoff
This site from ScienceWorld provides a short but factual biographical sketch of the German physicist, Gustav Kirchhoff and describes his most notable discoveries and contributions to the realm of science.